The present invention relates to valve gear for internal combustion engines, that is to say inlet valves and exhaust valves together with the associated actuating mechanism, and is concerned with such valve gear which includes a variable timing mechanism.
It is known that it is beneficial to vary the timing of the exhaust valves and, more particularly, of the inlet valves of an internal combustion engine. At high engine speeds, when the gases flow into and out of the cylinder at high speeds, it is beneficial to open the inlet valve earlier in the engine cycle and close it later in the engine cycle. This maximizes the quantity of inlet gas charge, i.e. fuel and air, entering the cylinder during the inlet period and takes advantage of the inertia of the charge whereby there is relatively little tendency of the inlet charge to flow directly from the inlet port out of the exhaust port.
At low engine speeds, later opening of the inlet valve is desirable to minimize the amount of inlet charge flowing directly into the exhaust port from the inlet port. This bypassing flow reduces engine power and efficiency by reducing the mass of the charge left in the cylinder for combustion, and increases the amount of hydrocarbons in the exhaust gas because the inlet charge contains unburned fuel. Earlier closing of the inlet valve at low engine speeds is desirable to minimize the amount of inlet charge flowing back into the inlet port during the compression stroke.
Many mechanisms have been proposed which provide variable valve timing (VVT), but they all have major limitations.
A known VVT mechanism, which is of the type disclosed in GB-A-170877, is illustrated diagrammatically in FIG. 1. This includes first and second cams 1 and 5 mounted on respective, parallel cam shafts 2 and 6. Spanning the two cams is a beam 9, which may also be described as a cam follower. The follower includes a respective cam-contact pad 4 and 8 at each end, which contact the cams at points 3' and 7' on respective cam-contact surfaces 3 and 7.
The follower 9 is centrally pivoted on a pin 150 which connects it to a lateral-restraint rod 152 pivoted about a fixed point 151. The rod 152 has a valve-contact pad 153 which is in contact with the end 10 of the valve member 13 of an engine inlet valve. The valve member is biassed towards the closed position by a spring 12, which acts on it via a spring retainer 11. Hence, any net downward movement of the follower creates a downward movement of the valve member, which is resisted by the spring.
The axis 14 of the valve member 13 is midway between the axes of the cam shafts 2 and 6. Therefore the lift or displacement at the valve at any time is equal to half the lift of cam 1 plus half the lift of cam 5 at that same time. This is illustrated diagrammatically in FIG. 2, which shows lift against time and in which the two cams are in phase. In FIG. 2a, L1 represents half of the lift of cam 1. In FIG. 2b, L2 represents half of the lift of cam 5. In FIG. 2c, L1+L2 represents the lift of the valve which lasts for a time Pa and rises to a maximum of La.
FIG. 3 is similar to FIG. 2 and shows the effect if cam 5 is given a phase shift S so that it is retarded relative to cam 1. The effect on valve lift is shown in L1+L2 in FIG. 3c. It will be noted that the maximum lift Lb of the valve is less than the maximum lift La in FIG. 2c when the cams are in phase, and the valve period Pb is greater than the original period Pa.
Thus, the valve opening period and lift are varied by advancing or retarding the phase of one camshaft relative to the other camshaft.
A typical cam for an internal-combustion engine has the characteristics shown in FIG. 4. FIG. 4a shows the curve of lift or displacement, L, of the valve on the Y axis against time on the X axis. During the cam period de the lift curve is concave. During the cam period efg the lift curve is convex. During the cam period kh the lift curve is concave. The lift is a maximum at point j, corresponding with point f on the period axis.
The corresponding diagram of velocity V is shown in FIG. 4b. During the opening period def the valve reaches its maximum opening velocity at 1. During the closing period fgh the valve reaches its maximum closing velocity at m.
The corresponding acceleration A is shown in FIG. 4c. During the period de the valve experiences a positive acceleration which reaches its maximum at n. During the period efg the valve experiences a negative acceleration which reaches its maximum at o. During the period gh the valve experiences a positive acceleration which reaches its maximum at p.
When two cams of this type are used in valve gear of the type illustrated in FIG. 1 the results are as shown in FIGS. 2 and 3.
Although, when the cams are out of phase, the lift curve is apparently smooth, there is a dynamic problem which is clearly evident if the acceleration diagram, FIG. 4c, is considered. Since the inertia forces involved are directly proportional to the magnitude of the accelerations, the acceleration curves are a good indicator of the VVT's ability to operate at high speed. At high speeds, excessive inertia forces cause excessive vibration and excessive mechanical stresses.
FIG. 5 shows the case where the two cams are out of phase by 12.5% of the cam period. FIG. 5a shows the acceleration curves of the two cams in this condition. FIG. 5b shows the acceleration of the valve. The new period is dte"f"g"wh", which is 12.5% longer than when the cams are in phase. Acceleration is defined by the curve dstue"o"g"vwxh". Points suvx identify four acceleration peaks, which are likely to cause severe vibration problems.
Clearly, it is not possible to use conventional cams at high speeds in a VVT of this type.
FIG. 6 shows the characteristics of the cams of the valve gear of FIG. 1 which attempt to overcome this problem. FIG. 6a shows the lift characteristics of the first cam. FIG. 6b shows the lift characteristics of the second cam. FIG. 6c shows the valve lift characteristic which occurs when the two cams are out of phase as shown. The first cam has a dwell period bd during which the lift of the cam remains constant, as shown by the line jq. The second cam has a dwell period fh during which the lift of the cam remains constant, as shown by the line rp.
It should be noted at this point that backlash is a small degree of clearance between the moving components when the valve member is not being activated, i.e. when the valve is closed. Backlash is necessary to allow the individual components to undergo thermal epansion when the engine is hot without opening the valve unintentionally during its closed period. Free movement is relatively large scale movement of the components in VVT's of the type described in which the components are separated by an amount greatly in excess of the backlash during some part of the valve operating cycle. Free movement is sufficiently large to preclude the use of conventional means of controlling backlash.
The valve gear shown in FIG. 1 includes a large free movement, shown as X in FIG. 6c, between the follower and the valve stem or between the follower and the cams. The lift L1 generated by the first cam is sufficient only to take up this free movement. This is illustrated by the line aj in FIG. 6c. At time c in the valve period the second cam begins to move the follower and, after taking up any backlash in the system, adds its lift L2 to the lift generated by the first cam. The result is valve lift, as shown by the cross-hatched area in FIG. 6c. By changing the phase angle between the two cams the magnitude and period of this lift may be varied.
As mentioned earlier, free movement is an undesirable characteristic in a high speed mechanism and many attempts have been made to overcome the problem. A notable example is disclosed in SAE paper 890676 which describes an improved VVT mechanism of the general type illustrated in FIG. 1. However, like all its predecessors the mechanism is bulky relative to a conventional fixed-period direct-attack valve mechanism and is not suited to high speed operation. These problems are mainly due to the fact that the mechanism incorporates a free movement compensation device to take up the free movement inherent in the system.
It is thus an object of the present invention to provide valve gear of the type including a VVT mechanism operated by more than one cam per valve in which the free movement or clearance between the components is substantially zero thereby permitting a backlash adjuster or compensator to be used, if desired, and also permitting the valve gear and thus the associated engine to be run at high speeds without the generation of substantial shock loads and noise.
According to the present invention there is provided a valve gear for an internal combustion engine including a first cam mounted to rotate about a first axis, a second cam mounted to rotate about a second axis which is substantially parallel to the first axis, a phase-change mechanism arranged selectively to vary the phase of one of the camshafts relative to the other, a valve member movable along a valve axis, biassing means urging the valve member in a first direction along the valve axis and a cam follower which has first and second contact surfaces arranged to be engaged by the first and second cams, respectively, and is arranged to transmit movement from the cams to the valve member, but is movable with respect to the valve member, characterised in that the profile of the first cam includes an ascending portion to move the valve member in a second direction opposite to the first direction and a descending ramp to control movement of the cam follower with respect to the valve member, that the profile of the second cam includes a descending portion to control movement of the valve member in the first direction and an ascending ramp to control movement of the cam follower with respect to the valve member, that the gradient of the ascending ramp and of the descending ramp are substantially the same over at least part of their length and that the phase of the two camshafts is such that the times for which the ascending and descending ramps contact the follower at least partially overlap at a time during which the valve member is stationary in the closed position and the cam follower moves with respect to the valve member.
In the construction illustrated in FIG. 1, when the valve member is in the closed position and can move no further in the closing direction due to its engagement with the valve seat, one or both of the cams moves away from the follower and the free movement appears and must be taken up again at a later stage in the cycle before the valve member can be moved by the cams. However, in the present invention, when the valve is closed the follower is engaged by the ascending and descending ramps whose gradient is the same and whose maximum height is preferably also the same which means that their combined height, that is to say the combined lift which they exert on the cam follower, remains constant and as the descending ramp tends to move away from the follower the follower is constrained to move with it by the ascending ramp. This means that the free movement does not open up at all and that the cams remain in constant contact with the follower at all times or spaced from it by a distance which is only the usual backlash distance which can be compensated for by a backlash adjuster.
In other words, the action of the ascending and descending ramps cancels out during the overlap period and the axial position of the follower along the axis of the valve member remains substantially constant and the cams remain substantially in contact with the cam follower which is therefore caused to move by the engagement of the cams. Hence the free movement is substantially zero during this period.
However, the axial position of the follower during the overlap period is dependent on the relative phasing of the two camshafts. Hence the backlash in the system varies with the varying phasing of the camshafts. For this reason it is preferred that there is a backlash adjuster, preferably of hydraulic type, e.g. acting on a tappet whereby any potential backlash in the system may be reduced to zero regardless of the phase of the camshafts.
In a wholly mechanical system in which there is no backlash adjuster or compensator, the valve motion may be optimised if the profile of each cam includes a portion of zero gradient adjacent the portion of maximum and of minimum lift.
The two cams may be carried by respective parallel cam shafts or by a single cam shaft associated with the phase change mechanism. In this case the axes of the two cams will be coincident and the follower may be of generally V shape or trough shape with outwardly inclined sides which afford the contact surfaces.
The cam follower may be of the type illustrated in FIG. 1, i.e. with the two contact surfaces generally coplanar, and in this event the movement of the follower relative to the valve member when the valve is closed will be pivotal movement. One disadvantage of this arrangement is that the lift of the valve is equal to the lift of each individual cam, assuming that the lift on each cam is the same. The volume occupied by the camshafts is however double that of a single direct-attack camshaft. Another disadvantage is that the pivot 150 is very highly loaded and rod 151 is needed for lateral location of the follower beam. This rod is bulky and adds to the weight, cost and space of the valve gear.
However, in a preferred embodiment in which the cams may be smaller than in the construction of FIG. 1 and the lateral location rod may be omitted entirely, whereby the valve gear is cheaper, lighter and simpler, the first and second contact surfaces are inclined to the valve axis by an acute angle of 15.degree. to 70.degree., the cam follower is movable transverse to the valve axis by the engagement of the cams and is constrained to move only substantially parallel to the valve plane in which the valve axis lies and which extends perpendicular to the two cam shafts.
The follower may act directly on the valve member but it is preferred that it acts indirectly via a tappet. The follower may be restrained to move parallel to the valve plane, and preferably substantially in the valve plane, by numerous means, e.g. by being restrained in a groove or recess in the cylinder head or an additional plate or by the provision of one or more flanges on the follower or tappet which engage in grooves or over the edges of the tappet or follower. A further possibility is that the tappet has an upstanding boss of at least part-circular section which is received in a groove in the underside of the follower whereby a degree of rotational movement of the follower or tappet is possible.
In its simplest form the cam follower is in the shape of a triangular prism with two planar surfaces which constitute the cam contact surfaces. However, these two surfaces may be either concave or convex. Alternatively, the cam follower may be of more complex shape but preferably still includes two cam contact surfaces which are inclined to one another and to the valve axis. In a further possibility, the cam contact surfaces are constituted by rollers or the like carried by the follower and in this event the actual shape of the follower is of no importance. If the cam contact surfaces are circular or arcuate it will be appreciated that what is of importance is that, when viewed in the direction of the length of the camshafts, the tangents at the points of contact of the cams and the contact surfaces, are inclined to the valve axis by 15.degree. to 70.degree..
Alternatively, the cam follower may be generally in the shape of a letter V including two divergent lugs whose opposed surfaces constitute the contact surfaces. In this event, the cams will not be carried by separate cam shafts but on a single cam shaft. The opposed contact surfaces will in practice not be directly opposed but will be slightly offset in the direction of the length of the cam shaft so that they can be engaged by the respective cams. This can tend to cause the cam follower to rotate about the valve axis and thus a further possibility is to divide one of the two cams into two halves which have the same shape and angular position and are spaced apart and separated by the other cam. In this case, the construction of the cam follower will be modified similarly and one of the inclined lugs is also divided into two halves which are spaced apart in the direction of the cam shaft, the other lug being opposed to the gap between the two halves. This arrangement ensures the elimination of the tendency for the cam follower to rotate.
The invention also embraces an engine including one or more cylinders, the inlet and/or outlet port(s) of which are controlled by such valve gear.
Further features and details of the invention will be apparent from the following description of certain specific embodiments.